1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method.
2. Description of Related Art
An electrophotographic method is a printing method, in which a laser light is irradiated on a photoconductor to form an electrostatic latent image, and a toner image formed on the photoconductor by development processing is transferred onto a sheet. A pulse width modulation (PWM) of the laser light is controlled according to a pixel value of an image to be formed, so that an irradiation range of the laser light is changed.
A multivalued dither method has been used as a reproduction method of a gradation when performing the printing by the electrophotographic method (see for example, Japanese Patent No. 4023095; and a non-patent document: Daniel L. Lau and Gonzalo R. Arce, “Modern Digital Halftoning”). The multivalued dither method is a method to convert an input pixel value of an image to an output pixel value of a plurality of levels, that is to say, to a multivalued output pixel value. For example, subcells are used in each of which two threshold values T1, T2 (wherein T1<T2) are set according to a position of pixels, so that the threshold values T1, T2 corresponding to the position of each pixel in the image are obtained by the subcells. Next, two threshold values T1, T2 and a pixel value are compared, so that the pixel value is converted to an invariable value Dmin when the pixel value is smaller than the threshold value T1, and the pixel value is converted to an invariable value DMax when the pixel value is larger than the threshold value T2. Further, the pixel value is converted to an interpolated value between the invariable values Dmin and DMax, when the pixel value is in the range of equal to or more than the threshold value T1, and equal to or less than the threshold value T2.
The resolution of an image has been advanced in recent years, thus there may be cases in which a beam spot diameter of the laser light happens to exceed the controllable size of one pixel. For example, as shown in FIG. 14, the size of one side of one pixel in a high resolution 1200 dpi is approximately 21 μm, whereas the beam spot diameter of the laser light is larger, which is approximately 60 μm. The circles drawn in a broken line in FIG. 14 respectively indicate the irradiation range of the laser light. As shown in FIG. 14, the beam spot diameter has the size covering a plurality of pixels, thereby the laser light irradiated to a focused pixel also happens to irradiate the pixels surrounding the focused pixel, which results in generating a dot gain.
FIG. 15 is a diagram describing the dot gain.
As shown in FIG. 15, in a case of an image in which the pixel value of the focused pixel has the maximum value of 255, and the pixel value of the surrounding pixels has the minimum value of 0, the laser light is emitted with the maximum output to the focused pixel having the maximum value of 255. When the light is emitted with the maximum output, the latent image formed by the laser light is to have an energy distribution with a broad base, thereby the size of each dot which forms the latent image is to be extended. As a result, although an exposure is performed for one pixel, a large dot which covers the plurality of pixels is formed, to generate an exceeded dot gain. Since the beam spot diameter exceeds the controllable size of one pixel, the dot gain is generated in the same manner also in the case where the pixel value of the focused pixel is a halftone of 128, thereby a dot exceeding the size of one pixel is formed, as shown in FIG. 15.
Such a dot gain invites a gradation collapse. This is because even when the pixel value of the focused pixel is a halftone, in a case where the adjacent pixels are of approximately the maximum value, the focused pixel is exposed when the adjacent pixels are exposed, thereby the adjacent pixels reach the maximum value.
As such, the reproduction property of the gradation in the original image has been reduced due to the generation of the dot gain.
When the multivalued dither processing is performed by a method in which a plurality of the subcells in combination, which is referred to as a supercell, are used in order to realize a further multiple gradation as shown in the Japanese Patent No. 4023095, the granularity may be reduced due to the dot gain and the gradation collapse. The pixel which is allotted with a small threshold value among the supercell is likely to reach the maximum value, thereby the dot gain is generated, so that a large halftone dot is to be formed in the subcell including such a pixel. On the other hand, in the pixel which is allotted with a large threshold value, the pixel value of the minimum value or the halftone is output, thereby the dot gain is not generated (or the dot gain is generated in a small degree, if any) in the subcell including such a pixel, so that a small halftone dot is to be formed. As such, even when the density of an image is even, the image gives the impression of having an uneven granularity due to the large and small different halftone dots being mixed therein.
The above problem may be solved when a laser light having a beam spot diameter smaller than the size of one pixel with a high resolution is used, however, such a laser light source of an enhanced performance is expensive and demands cost.
Further, even in a case where the beam spot diameter is not larger than the controllable size of one pixel, when the laser light is output with a large power such that the light amount given to one pixel is equivalent to that given to a beam spot diameter which is larger than one pixel, the same problem presumably occurs. When the power is large, the energy distribution in the latent image is to be extended accordingly, which results in the extension of the size of the dots to be formed.